Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details)

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Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($)
$ in Millions
Nov. 30, 2020
May 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted and unrestricted cash equivalents $ 17.3 $ 43.5
Available-for-sale securities 3,186.8 2,757.2
Other 32.0 27.2
Other long-term liabilities 30.1 26.8
Asset-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 77.7 69.7
Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 665.7 683.6
Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 1,744.0 1,409.6
U.S. Government Agency And Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 556.5 594.3
Money Market Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted and unrestricted cash equivalents 17.3 43.5
VRDNs [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 142.9  
Quoted Prices in Active Markets (Level 1) [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted and unrestricted cash equivalents 17.3 43.5
Other 32.0 27.2
Other long-term liabilities 30.1 26.8
Quoted Prices in Active Markets (Level 1) [Member] | Money Market Securities [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted and unrestricted cash equivalents 17.3 43.5
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 3,186.8 2,757.2
Significant Other Observable Inputs (Level 2) [Member] | Asset-Backed Securities [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 77.7 69.7
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 665.7 683.6
Significant Other Observable Inputs (Level 2) [Member] | Municipal Bonds [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 1,744.0 1,409.6
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agency And Treasury Securities [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 556.5 $ 594.3
Significant Other Observable Inputs (Level 2) [Member] | VRDNs [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities $ 142.9